Measuring Specular Gloss Changes during Standard Paperboard Rub Tests
Measuring specular gloss changes before and after standard rub tests quantifies surface coating durability using exact optical angle decay metrics.
Surface Scatter
Optical Dispersion
Reflective deflection of incoming light rays at substrate boundary interfaces governs brightness and surface gloss characteristics of paper materials. The phenomenon known as surface scatter occurs when light encounters refractive index micro-discontinuities between cellulose fibers and mineral coating pigments. High surface scattering increases sheet brightness and hiding power by reflecting light before it penetrates into the paper matrix.
Spectrophotometers measure directional surface reflections to quantify gloss and surface micro-texture.
Refractive Kinematics
Uncoated paper webs contain irregular surface topography that scatters light diffusely in all directions. Mineral coatings containing calcium carbonate or titanium dioxide introduce fine particles that increase diffuse scatter across visible wavelengths. Calendering smooths surface micro-roughness, reducing diffuse scatter and raising specular gloss.
Substrate Influence
Pigment particle size distribution controls light scattering efficiency according to Mie scattering principles. Particles sized near half the wavelength of visible light maximize light reflection and sheet opacity. Excessive binder content in coating formulations fills air voids between pigment particles, reducing refractive index variations and lowering surface scatter.
Moisture application during calendering flattens surface features and lowers light reflectance. Ink vehicle absorption alters coating porosity, changing scatter properties in printed areas compared to unprinted margins. Reflectance measurements verify coating uniformity across paper mill production runs.